A sense amplifier circuit is an integrated circuit that is typically used to measure the amplitude and direction of current in a digital circuit. A sense amplifier circuit can be used within a number of different digital circuit applications. For example, a sense amplifier circuit can be used to read out data stored in an EEPROM (electrically erasable programmable read-only memory).
FIG. 1 shows a conventional sense amplifier circuit 100 including a sensing circuit 102 for sensing a bit from a memory cell 104 to extract a value stored within the memory cell 104. If the bit within the memory cell 104 has been erased (e.g., logic 1), then the current Icell delivered by the memory cell 104 is zero. If the bit within the memory cell 104 has been previously written to (e.g., logic 0), then the current Icell is not zero (e.g., 15 μA). The sensing circuit 102 compares the current Icell to a threshold current Ithr (which can be, for example, 5 μA). If the current Icell is lower than the threshold current Ithr, then the node (DOUT) goes high. In an opposite manner, if the current Icell is higher than the threshold current Ithr, then the node (DOUT) goes low. The limitation, however, of the sense amplifier circuit 100 is that the threshold current Ithr varies nonlinearly—e.g., as a square law—with respect to the supply voltage VDD. Such a nonlinear behavior can causes the current threshold Ithr to become very low at low supply voltage levels (e.g., VDD near 0V) which can drastically increase access time of the memory cell 102. Likewise, the current threshold Ithr can be become too high at high supply voltage levels (e.g., VDD near 2V), which can cause the sense amplifier circuit 100 to fail.